text: When writing a Clar structure, the following rules must be satisfied:
  #each  vertex of the molecular graph representing the polycyclic aromatic hydrocarbon either belongs to a double bond or a circle;
  # such double bonds and circles never join;
  # there are no rings with three double bonds, since they are always represented by circles; moreover, the number of circles in the graph is maximized;
  #when a ring with a circle is adjacent to a ring with two double bonds, an arrow is drawn from the former to the latter ring. 
  Some observations about these rules are worth to be put into evidence. Following Clar, rules at points 1 and 2 imply that circles can never be in adjacent rings; rule at point 3 means that only four options are viable for rings, namely (i) having only one double bond, (ii) having two double bonds, (iii) having a circle, or (iv) being empty, i.e. having no double bonds; finally, the arrow mentioned in the rule at point 4 can be interpreted in terms of mobility of π-sextets (in this case we speak of migrating π-sextets) or, equivalently, of a  quantum-mechanical  resonance between different Clar structures.